Energy filter image generator for electrically charged particles and the use thereof

ABSTRACT

The invention relates to an energy filter image generator for filtering electrically charged particles. The inventive energy filter comprises at least two toroidal energy analysers ( 30, 40 ) arranged one inside the other. A transfer lens device ( 20 ) is disposed between the plane of emergence ( 5 ) of the first energy analyser ( 30 ) and the plane of incidence of the second energy analyser ( 40 ), thereby making it possible to obtain the perfect energy filtered reproduction of the surface ( 1 ′) of a sample on a detector ( 10 ).

RELATED APPLICATIONS

This application is a national stage application based on InternationalApplication No. PCT/EP2003/012283, filed Nov. 4, 2003, which claimspriority from German application No. 102 52 129.8 filed Nov. 4, 2002.

BACKGROUND OF THE INVENTION

The invention pertains to an image-generating energy filter forelectrically charged particles such as electrons and ions with at leasttwo toroidal energy analyzers arranged in a row, where at least oneenergy analyzer has a diaphragm in its entrance plane and anotherdiaphragm in its exit plane. The invention also pertains to the use ofthese image-generating energy filters.

The diaphragms in the entrance and exit planes can be slit diaphragms orcircular diaphragms perpendicular to the associated energy-dispersiveplane.

The term “energy filter” is understood to mean preferably an imaging orimage-generating energy filter. The use of imaging filters is especiallyadvantageous when the image fields being processed in parallel containmore than 100×100 pixels. The recording times are then much shorter thanthose of a spectrometer, which scans the sample sequentially.

Energy filters are used in, for example, photoelectron spectroscopy,which is one of the most important methods of the quantitativeelementary analysis of surfaces. Measuring the energy distribution ofphotoelectrons with high local resolution is called spectromicroscopy.There are essentially two different methods which can be used to achievea high degree of local resolution.

In the first variant, the sample is scanned by a focused photon beam,and the energies of the photoelectrons coming from the individualemission spots thus defined are analyzed.

In the second variant, the photon beam is focused just long enough toilluminate the visual range of the objective lens. Electron-opticalmeans are then used to produce a magnified image of the intensitydistribution of the generated photoelectrons.

To derive a map of the distribution of the elements or of the chemicalbonds, the kinetic energies of the photoelectrons must be analyzed.Various techniques have been developed to accomplish this intransmission-electron microscopy. Here, too, there are essentially twodifferent principles:

There are microscopes which use all of the electrons to generate animage. A small percentage of the electrons pass through an energyanalyzer to generate a spectrum of a portion of the image. In anotherpart of the microscope, only a narrow energy band is processed, but acomplete image is transported through the energy analyzer.

The electrons are filtered by electrostatic or magnetic devices, whichallow only the electrons with a certain energy to pass through. Theintensity of the resulting beam reflects the concentration of a chemicalcomponent present on the surface of the sample. In this method, it isimportant for the local resolution not to deteriorate as the beam passesthrough the monochromator.

Several different energy analyzers have been developed to perform thisimaging function. Because of its good transmission and energyresolution, the hemispherical analyzer has become widely accepted incommercial devices for energy analysis not requiring image quality.

The possible imaging properties of electrostatic energy analyzers werestudied many years ago on the basis of analyzers with general toroidalfields (B. Wannberg, G. Engdahl, A Sköllermo: Imaging properties ofelectrostatic energy analyzers with toroidal fields, J. Electron Spectr.Rel. Phenomen. 9 (1976), pp. 111-127). For a toroidal potential, theradius of curvature in a first direction is different from that in asecond direction perpendicular to the first. A spherical capacitor witha ratio of 1 between the radii is included as a special case in thisgeneral description. A cylindrical capacitor is curved in only onedirection, and the ratio between its radii is zero. Some spectrometersmake it possible to adjust the transition between the field forms in acontinuously variable manner, as described, for example, in K. Jost:Novel Design of a spherical electron spectrometer, J. Phys. E.: Sci.Instr., 12, 1979, pp. 1006-1012.

An electron microscope with an energy filter comprising a sphericalanalyzer of hemispherical design is known from EP 0,293,924 B1. Toimprove the imaging quality of the energy filter, a complicated lenssystem is set up in front of the entrance slit so that the arrivingelectron beams are as close to perpendicular as possible. For electronswhich start at the mean path radius r₀=x₀, it should be true thatα₀=−α₁, where α₀ stands for the angle at the entrance to the energyfilter and α₁ for the angle at the exit.

It is claimed that the entrance angles of these electrons aretransferred exactly to the exit angles regardless of their energy.

To take advantage of this property, a magnified image of the sample isplaced not at the entrance slit of the analyzer but rather at the focalpoint of a lens, which is set up in front of the slit diaphragm of theanalyzer. Thus the position of the image is transformed into angles. Theentrance slit diaphragm is placed on the image side of the lens at thefocal point.

The exit slit of the analyzer selects the desired energy range. Anotherlens behind the analyzer reconstructs a now energy-filtered local imagefrom the transmitted angle image. This can be magnified further and madevisible on a screen with the help of an intensity amplifier, such as amicrochannel plate.

An electron spectrometer with a similar arrangement is described in EP0,246,841 B1. A local resolution of down to 2.5 μm is obtained with thisenergy analyzer of the toroidal capacitor type, which has a lens systemin front and another behind.

It was overlooked, however, that the equation α₁=−α₀ is usually only arough approximation. In Nucl. Instr. Methods A291 (1990), pp. 60-66, itis shown that the entrance and exit angles also depend on the entranceand exit locations. The entrance and exit angles will differsignificantly from each other when the entrance and exit positions aredifferent. It is then true that (tan α₀):x₀=−(tan α₁):x₁.

The aberrations increase with the size of the magnified image field,that is, with the possible difference between x₁ and x₀. The followingexample can illustrate the magnitude of these defects:

In the case of a visual field with a diameter of 4 mm, where, forexample, x₀=122 mm and x₁=126 mm, and for an acceptance angle of α₀=5°,we can calculate an exit angle of α₁ of 5.16°. This is a 3% deviationfrom the incidence angle. In the case of a visual field with a radius of100 μm, this results in an imaging error of 3 μm at the edge of theimage field.

Electrons with the same entrance position but different entrance anglesalso have different exit positions and different exit angles accordingto:

${\tan\mspace{11mu}\alpha_{1}} = {\tan\mspace{11mu}{{\alpha_{0}\left( {1 - \frac{2}{\cos^{2}\alpha_{0}}} \right)}^{- 1}.}}$

This is described in, for example, T. Sagara et al., ResolutionImprovements for hemispherical energy analyzers, Rev. Sci. Instr. 71,2000, pp. 4201-4207.

In another example, a hemispherical analyzer is used in a differentoperating mode. Here the potentials are selected so that the electronstravel along a hyperbolic path in a field which rises with the square ofthe-radius.

U.S. Pat. No. 5,185,524 describes an electrostatic analyzer withspherical mirrors. The electrons pass into the inner sphere throughslits and are brought back out through the inner sphere to a focal pointby an opposing field. Both the object and the image are located insidethe inner sphere.

The disadvantages of this arrangement are described in Nucl. Instr.Methods 42, 1966, pp. 71-76. Large slits are present in the inner sphereat locations where the cross section of the beam is not small. Pieces ofnetting are attached at these points to ensure the required sphericalpotential. Only a portion of the field passes through the mesh, whichlimits the local resolving power. Each mesh opening represents a smalldiverging lens. Another disadvantage of using netting in the path of thebeam is the production of secondary electrons, which leads to anincrease in background noise and thus reduces the displayable contrast.The energy-selecting slit is located in the electrical field between thehemispheres and is therefore difficult to reach and adjust. The voltageswhich must be applied to the outer sphere are much higher than thoserequired for the conventional hemispherical analyzer.

In this design, as also in the preceding one, there are inherentaberrations, which can be attributed to the merely two-fold symmetry ofthe instrument's construction.

DE 196 33 496 A1 describes a monochromator for electron microscopy withmirror symmetry. The design in the form of a Ω avoids second-orderaberrations, and even some of the third-order aberrations disappear. Oneof the essential criteria for the selected design was the avoidance ofan intermediate focus. The goal here is to make it possible tomonochromatize a primary electron beam of small diameter and highcurrent density. This requirement leads to a complicated mechanicalsolution. The design consists of eight toroidal sectors, which must beadjusted very precisely with respect to each other. The device istherefore very costly to make and very time-consuming to adjust.

A similar mirror-symmetric arrangement of monochromators is selected inEP 0,470,299 A1. This arrangement also lacks an intermediate lens, butit does have a straight connecting tube. The energy-selecting slit islocated in the plane of symmetry. No provisions are made for generatingimages in this case, either.

An energy filter consisting of a complementary opposing pair of 90°sectors, which are arranged with respect to each other in such a waythat they form an “S”, is known from U.S. Pat. No. 5,466,933. Anaperture diaphragm is set up between the two sectors. With this energyfilter, an image of the incoming parallel electron beam is produced atthe exit from the sector arrangement.

Although this arrangement using parallel electron beams does make itpossible to obtain a high-contrast image at the exit of the energyfilter, the intensity present at the exit is extremely low. Theintensity can be increased by allowing electrons with an entrance angleao not equal to zero to enter as well, but then the pixels are smearedand the contrast is reduced.

WO 01/61,725 A1 describes an emission electron microscope, whichcontains an image-generating beam path consisting of an electron-opticimaging system, which subjects the electron beam to a parallel shift andanalyzes its energy. It consists of two spherical energy analyzers witha lens inserted between them. This lens is located at the focal point ofthe two analyzers. An intermediate image of the sample or the angleimage of the sample is placed at the center of this lens. Because themagnification of field lenses is positive, aberrations which arise onpassage through the first deflector are not corrected. This documentdoes not mention or discuss the correction of aberrations.

DE 3,014,785 A1 describes a double monochromator for charged particles,which contains a delay lens in the form of slit diaphragms between thetwo monochromator subunits. The monochromator operates without loss ofenergy resolution at higher intensities than was possible in the past.No lens which might improve the imaging properties of the system ismentioned. Slit diaphragms are also described in U.S. Pat. No.4,742,223. The imaging properties of the system are not discussed.

U.S. Pat. No. 5,448,063 describes an image-generating, mirror-symmetricenergy filter, which compensates only for 2^(nd) and 3^(rd)-orderaberrations. This defect correction is achieved only by the use ofcomplicated equipment, which includes additional hexapole fields.

SUMMARY OF THE INVENTION

The task of the invention is therefore to create an image-generatingenergy filter with minimal aberration, which guarantees both ahigh-contrast image with high local resolution and high intensity at theexit.

This task is accomplished by an energy filter which is characterized inthat a transfer lens device with negative lateral magnification V_(L),negative angle magnification V_(W), image rotation by the angleγ=β−180°, and a telescopic beam path is placed between the exit plane ofthe first energy analyzer and the entrance plane of the second energyanalyzer, where all the deflection angles φ of the transfer lens are thesame, and where its energy-dispersive planes (33, 43) are rotated aroundthe angle β with respect to each other.

The energy analyzers are rotated around the axis of the transfer lensdevice.

A “telescopic beam path” is understood to be a beam path in which thevarious clusters of parallel beams are converted to a single parallelbeam cluster regardless of their angles of incidence. The angle γ standsfor the degree to which the image is rotated from its inverse position.This inverse position is achieved by the use of, for example,electrostatic lenses.

The advantage of the energy filter is that it is not mandatory to workwith particle beams arriving in perpendicular fashion at the entranceplane of the first energy analyzer; that is, the entrance angle α₀ canbe allowed to be unequal to zero, which means that a high-contrast imageof high intensity can be produced at the exit of the energy filter. Theaberrations present at the exit from the first energy analyzer,especially the second-order aberrations, are transformed by the transferlens device and projected onto the entrance plane of the second energyanalyzer in such a way that that these aberrations are completelyeliminated when the charged particles travel through the second energyanalyzer.

The image quality of the energy filter is limited essentially only bythe quality of the transfer lens device.

The transfer lens device is preferably designed so that, in theenergy-dispersive plane, it projects the intermediate image ZB₁ presentat the exit plane of the first energy analyzer with a linearmagnification of

$V_{L} = {\frac{{ZB}_{2}}{{ZB}_{1}} < 0}$and with an angular magnification of

${V_{W} = {\frac{\alpha_{2}}{\alpha_{1}} < 0}},{{{where}\mspace{14mu} V_{W}V_{L}\sqrt{\frac{E_{2}}{E_{1}}}} = 1},$rotated around the angle γ=β−180°, onto the entrance plane of the secondenergy analyzer as an intermediate image of the size ZB₂, where α₁ isthe exit angle of the charged particles from the exit plane of the firstenergy analyzer; α₂ is the entrance angle to the entrance plane of thesecond energy analyzer; E₁ is the kinetic energy of the chargedparticles in the exit plane of the first energy analyzer; E₂ is thekinetic energy of the charged particles in the entrance plane of thesecond energy analyzer, and where the charged particles travel through atelescopic beam path in the transfer lens device.

For an electrostatic transfer lens device, β=180° and V_(L) and V_(W)are negative. For a magnetic transfer lens device, V_(L) and V_(W) arealso negative, but the image can be subject to rotation, which meansthat it is necessary to select β≠180°.

The energy analyzers and the transfer lens device are preferably set upwith point symmetric around the center Z of the transfer lens device.This means that the energy analyzers have the same constructiondimensions and that V_(L)=−1 and V_(W)=−1.

The energy analyzers can also have different construction dimensions, asa result of which an arrangement with quasi-radial symmetry is created,in which V_(W) and V_(L) are less than 0.

V_(W)=V_(L)=−1 can be obtained by adjusting the radii and the passenergies appropriately to each other.

Within the scope of the invention, the choice of the type of deflectionfields used for the analyzers is essentially free. Magnetic fields,either permanent or generated by electrical current, can be used, butelectrostatic fields are especially preferred.

The toroidal energy analyzers are preferably sectors of a sphere orcylindrical analyzers, especially with a deflection angle of more than90°.

Hemispherical analyzers with deflection angles of φ=180° are especiallypreferred, because these have an especially high energy dispersion attheir exit.

In a spherical field, in which the potential energy of the particle is˜1/r, the charged particles move along closed elliptical paths. All ofthe particles which start at one point, even though of differentenergies and even of different angles, reach their exact originalposition again after a circuit of 360°. As a result, there are noaberrations at the exit. In a closed spherical analyzer of this type,the two energy-dispersive planes of the two hemispheres would bydefinition enclose the angle β=0. After traveling 180°, the particleswith different energies reach their maximum distance from each other. Ifan aperture which does not disturb the radial field is placed here, onlythe particles of the desired energy are allowed to pass through.Nevertheless, in a closed spherical analyzer of this type, there is noroom to put an entrance lens and a detector, for example, or a transferlens.

In the case of two hemispherical analyzers which are arranged withrespect to each other in such a way that their energy-dispersive planesare rotated around the angle β, the imaging properties of the transferlens device makes it possible to retain the properties of a completespherical analyzer, so that defect-free images will be obtained at theexit of the energy filter. The transfer lens device ensures that thepaths are exact images of each other, the only difference being that theentrance point to the first hemispherical analyzer is separated in spacefrom the exit point of the second hemispherical analyzer.

The effect thus obtained is that of a spherical capacitor. It is knownthat non-relativistic particles travel along closed, periodic ellipticalpaths. The angles and positions are the same after a complete circuit.This is independent of the starting position, of the entrance angle, andof the energy of the charged particles.

The energy-dispersive plane of the hemispherical analyzers is preferablyrotated by an angle of β=180° around the axis of the transfer lensdevice, so that the beam path has the shape of an “S”. This arrangementoffers the advantage that an especially simple transfer lens device canbe used.

For practical reasons, the angle β between the dispersion planes ispreferably within the range between 5° and 355°, especially between 15°and 340°.

The energy analyzers can be of different designs; if so, the design ofthe transfer lens device must be modified accordingly. In the case ofdifferent energy analyzers, e.g., different pass energies E₁ and E₂, thelateral magnification and the angular magnification must be adjusted toproduce the desired intermediate image on the entrance plane of thesecond energy analyzer. In the design of the transfer lens arrangement,therefore, it is necessary to take in the account the Lagrange-HelmholtzequationZB ₁ ·α ₁ ·√{square root over (E₁)}=const.=ZB ₂·α₂ ·√{square root over(E₂)}.

From a cost standpoint, however, it is advantageous to use identicalenergy analyzers and to work with the same pass energies. In this case,the lateral magnification of the transfer lens device is V_(L)=−1, andthe angular magnification of the transfer lens device is V_(W)=−1.

The transfer lens device preferably comprises at least one electrostaticlens, especially an electrostatic tube lens, which is used especially inconjunction with two hemispherical analyzers with energy-dispersiveplanes which are preferably rotated with respect to each other by theangle of β=180°.

The transfer lens device can comprise at least one magnetic lens.Magnetic lenses offer the advantage that they produce smalleraberrations than electrostatic lenses. They are therefore preferred incases where, because the energy analyzers are rotated by the angle βfrom each other, the intermediate image ZB₁ must also be projected witha rotation around the angle γ=β−180°.

The transfer lens device preferably has at least two lenses.

It is advantageous to locate the exit plane of the first energy analyzerat the focal point of the first lens and to locate the entrance plane ofthe second energy analyzer at the focal point of the second lens, where2F stands for the distance between the two lenses and F stands for thefocal distance of the two lenses.

The transfer lens device can also have at least one electrostatic ormagnetic multipole lens. Multipole lenses offer the advantage that theycan provide an image without any spherical aberrations. A multipole lensis set up between the two energy analyzers in such a way that a radiallysymmetric arrangement is obtained.

According to one possible use of the energy filter, the filter is placedin the imaging beam path of an image-generating electron-optic system.The task of the energy filter is to select electrons of certain energiesfrom the beam path through appropriate adjustment of the diaphragms ofthe energy filter. It is irrelevant whether an intermediate image of thesample to be studied, the Fourier-transformed intermediate image, orsome other intensity distribution of the imaging beam path is sent tothe entrance slit of the energy analyzer. The energies of these chargedparticles and energy bandwidth of the detected beam path can bedetermined and varied by changing the energy window.

One of the preferred uses of the energy filter is in electronmicroscopy. Here the energy filter is used to produce an image of theelectrons emitted and back-scattered by an object. These electrons haveby nature a wide energy spectrum. The contrast can be improved by usingelectrons from a narrow energy band. By selectively setting the energywindow, a succession of specific signals can be selected out andamplified, while the others can be attenuated. It is therefore possibleto emphasize a certain set of data.

Another preferred use of the energy filter is in time-resolved measuringinstruments. The advantage of the energy filter is that even differencesin the times of flight which occur in the first energy analyzer areeliminated by the transfer lens device and passage through the secondenergy analyzer.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are explained in greater detailbelow on the basis of the drawings:

FIG. 1 shows a schematic diagram of an energy filter with twohemispherical analyzers;

FIG. 2 shows a schematic diagram of a transfer lens device;

FIG. 3 shows an embodiment of the arrangement shown in FIG. 1;

FIG. 4 shows a perspective view of the embodiment according to FIG. 3;

FIG. 5 shows another embodiment, which differs from that shown in FIG. 4by a different angle of rotation;

FIG. 6 shows a schematic diagram of another embodiment in whichspherical sectors are used as energy analyzers;

FIG. 7 shows another embodiment with a total of four toroidal sectors;

FIG. 8 shows an energy filter with cylindrical analyzers;

FIG. 9 shows a transfer lens device with multipole lenses; and

FIG. 10 shows a transfer lens device with magnetic lenses.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic cross-sectional diagram of an energy filter,which has two hemispherical analyzers 30 and 40, between which atransfer lens device 20 is located. The two energy analyzers 30, 40together with the transfer lens device 20 are set up in such a way thatthe beam path lies in a plane and has the shape of an “S”.

The overall arrangement has radial symmetry with respect to the center Zof the transfer lens device 20; the radial symmetry is two-fold.

The electrons curve to the left in the first energy analyzer 30, andafter they have passed through the transfer lens device 20, they curveto right in the second energy analyzer 40. This means that the twoenergy-dispersive planes 33, 43 of the two energy analyzers are rotatedby the angle β=180° with respect to each other (see FIG. 4).

FIG. 1 shows only the center beam paths 4 and 7 of the electrons in thefirst and second energy analyzers. The energy filter hasimage-generating properties while avoiding aberrations of the second andhigher orders.

The surface 1′ of the sample 1 is a certain distance g from the firstlens system 2, which forms an image of the electrons emerging from thesurface 1′ on the entrance plane 3 of the first hemispherical analyzer30.

The object distance g can be the same as the focal distance of the lenssystem 2, so that the image distance b is approximately equal toinfinity. In this case, the entrance plane 3 of the first energyanalyzer 30 is preferably located in the image-side focal plane of thelens system 2.

In the entrance plane 3 there is a first energy-defining slit diaphragm25, which is perpendicular to the plane of the drawing and has the widthB₁ (see also FIG. 3).

The hemispherical analyzer 30 forms an image of the electrons enteringthrough the slit diaphragm 25 with aberrations in the exit plane 5,where a second slit diaphragm 26 with the width B₂ is located.

Because the electrons enter the slit diaphragm 25 in the entrance plane3 at various angles α₀, they also exit at different exit angles α₁ uponleaving the deflection field of the first energy analyzer.

The second slit diaphragm 26 is perpendicular to the plane of thedrawing in which the linear focus of the astigmatic intermediate imageZB₁ 23 lies. The energy dispersion occurs in the plane of the drawing.This dispersion is defined as the deviation from the central beam path 4by a value which is proportional to the energy deviation. By changingthe width B₂ of the slit (see also FIG. 3), it is possible to adjust orto change selectively the energy bandwidth of the electrons let throughby the slit diaphragm 26. As a result, the only electrons which reachthe intermediate image ZB₁ 23 are those which lie within this energybandwidth. The electron beam is monochromatic as a result.

A transfer lens device 20 is set up behind this exit plane 5. Thisdevice consists of two identical converging lenses 21 and 22 and formsan image of the first intermediate image 23 produced in the exit plane 5as an inverted second intermediate image ZB₂ 24, that is, V_(L)=−1, atthe entrance plane 6 of the second energy analyzer 40.

The transfer lens device 20 not only inverts the intermediate image ZB₁23 on the entrance plane 6 but also inverts the angles, so that theentrance angles α₂ in the entrance plane 6 of the second energy analyzer40 are described by α₂=−α₁.

The aberrations are eliminated in the second energy analyzer 40 as aresult of the inversion of the astigmatism of the intermediate image ZB₁23 in conjunction with the inversion of the path curvature present inthe first energy analyzer 30. An energy-filtered, stigmatic image 29,which can be projected by the lens system 9 onto a detector 10, is thuscreated in the exit plane 8.

In this embodiment, the second energy analyzer 40 also has a slitdiaphragm 27 of width B₃ in the entrance plane 6 and a slit diaphragm 28of with B₄ in the exit plane 8.

If the distance of the surface 1′ of the sample 1 or of a magnified orreduced image is equal to the focal distance of the lens system 2, thedistance of the lens system 9 from the exit plane 8 will also be equalto the focal distance, and the distance to the detector 10 will be equalto the focal distance of the lens system 9.

Diffraction images instead of real images are then present at theentrance and exit planes of the two energy analyzers 30, 40. If the lenssystems 2 and 9 are operated asymmetrically, it is possible to use theenergy filter to obtain a diffraction image of sample 1 without anyaberrations of the second and higher orders. It is said that the lenssystems are operated “asymmetrically” when either the lens system 2projects the surface of the sample onto the entrance plane 3 and thelens system 9 is adjusted in such a way that the intermediate image 29is situated at the focal distance of the lens system 9, or converselythe lens system 2 is adjusted in such a way that the sample surface (orits intermediate image) lies in the focal plane of the lens andsimultaneously the lens system 9 projects the plane 8 sharply onto thedetector 10.

The diffraction image of the sample is then projected by the lens system2 onto the entrance plane 3. This diffraction image is energy-filteredand ultimately arrives at the exit plane 8. From there it is projectedby the lens system 9 onto the detector 10.

FIG. 2 shows a schematic diagram of the beam path in the transfer lensdevice 20. The two identical electrostatic converging lenses 21, 22 havean F-2F-F arrangement, where F is the focal distance of the lenses 21,22. On the basis of this lens arrangement, the first intermediate imageZB₁ 23 in the exit plane 5 with the lateral magnification V_(L)=−1 andthe beams with the angular magnification V_(W)=−1 are projected onto theentrance plane 6 as a second intermediate image ZB₂ 24. The beam path isradially symmetric and telescopic.

When other types of lenses are used, e.g., electron-optic cylindricallenses, the angular and lateral magnifications can also be +1 in thenon-dispersive plane.

FIG. 3 shows a possible embodiment of the arrangement illustratedschematically FIG. 1 with three possible electron paths E₀, E₁, and E₂.A cross section through the energy-dispersive planes is shown.

The electrons start from the surface 1′ of the sample 1, pass throughthe slit diaphragm 25 of width B₁, and enter the first hemisphericalanalyzer 30, in which an electrostatic deflecting field is appliedbetween the inner shell 31 and the outer shell 32.

When the electrons enter the slit diaphragm 25 at a right angle, as theydo at point X₀, they describe a path E₀, which describes a semicircle ineach of the first and second hemispherical analyzers.

Because the path E₀ meets the axis 200 of the transfer lens device 20,the electrons are also projected onto point X₀ of the slit diaphragm 27of the second hemispherical analyzer 40, and the path along which theytravel in the second hemispherical analyzer is radially symmetric topoint Z.

The electrons on path E₁ start at point X₁ of the slit diaphragm 25 ofthe first hemispherical analyzer 30 with a different energy and adifferent entrance angle α_(0,1), whereas the electrons of path E₂ startat point X₁ with the entrance angle −α_(0,2). The electrons aredeflected to point X₂ in the second slit diaphragm 26, describingelliptical paths in both cases. The exit angles are α_(1,1) and α_(1,2),where |α_(1,1)|=|α_(1,2)| was selected in this example.

The pixel X₀ of the first intermediate image ZB₁ in the slit diaphragm26 is projected with the lateral magnification −1 and with the angularmagnification −1 onto the plane 6 at point X₃ as a pixel of the secondintermediate image ZB₂. For the angles we therefore haveα_(1,2)=−α_(2,2) and α_(1,1)=−α_(2,1).

In the second energy analyzer 40, an equally intense electrostaticdeflecting field is applied between the inner shell 41 and the outershell 42, so that the electron paths E₁ and E₂ have elliptical courseswhich correspond to the elliptical paths in the first energy analyzer30. The electrons exit at point X₄ at the angles α_(3.1) and α_(3.2),which correspond in turn to the angles α_(0.1) and α_(0.2). Thedeviations of the angles are α_(1.1) and α_(1.2) are compensated by thesecond pass, i.e., by the pass through the energy analyzer 40. It isalso true with respect to the point X₄ that X₄=X₁. An energy-filteredimage of the sample 1 is thus obtained without aberration in the planeof the slit diaphragm 28.

FIG. 4 shows a perspective view of the embodiment shown in FIG. 3. Theenergy-dispersive planes 33 and 43 and the slit diaphragms 25, 26, 27,and 28 in the hemispherical analyzers 30, 40 are illustrated. The secondhemispherical analyzer 40 is rotated by the angle β=180° around the axis200 of the transfer lens device 20, which axis passes through the slitdiaphragm 27.

FIG. 5 shows another embodiment, in which the second hemisphericalanalyzer 40 is rotated by the angle of only β=90° around the axis 200passing through the slit diaphragm 27.

FIG. 6 shows an embodiment corresponding to that of FIG. 3, where,instead of the hemispherical analyzers 20, 30 [Sic; →30, 40-Tra],spherical sectors 20′, 30′ [Sic; →30′, 40′-Tra] are used, which haveinner shells 31′, 41′ and outer shells 32′, 42′ with deflection anglesof φ≦180°. The arrangement of the diaphragms 25, 26, 27 differs from thearrangement according to FIG. 3 in that they are not located in theentrance and exit planes of the spherical sectors. This embodiment alsoshows two-fold radial symmetry with respect to point Z.

FIG. 7 shows the arrangement according to FIG. 6 supplemented by twoadditional toroid sectors 50 a, 50 b. The toroid sector 50 a is placedin front of the first spherical sector 30′, and the toroid sector 50 bis placed behind the second spherical sector 40′. These additionaltoroid sectors 50 a, 50 b serve to correct higher-order aberrations.

FIG. 8 shows an energy filter consisting of two cylindrical analyzers30′, 40′ [Sic; →30″, 40″-Tra] with inner shells 31″, 41″ and outershells 32″, 42″ and a transfer lens device 20. The axis 200 of thetransfer lens system 20 is not collinear to the cylinder axes 34, 44 butextends instead in the direction of the central paths 4′, 7′ through thecylindrical analyzers, which form an angle of 42.3° with the cylinderaxes 34, 44.

FIGS. 9 a and 9 b show a transfer lens device 20 which avoids bothspherical aberration and the coma error. This can be achieved bycombining electrical or magnetic round lenses (21, 22) with twosextupole lenses 121, 122. The axis 200 of the transfer lens deviceextends in direction z.

FIG. 9 a shows a cross section through a sextupole segment perpendicularto its axis. The force F on a particle changes its direction between twoadjacent electrodes, the voltages U and −U relative to the axispotential being applied to alternate electrodes.

FIG. 9 b shows schematically the course of two electrons a certaindistance away from the axis. At the point of entrance, the axes of theseelectrons are parallel in the xy cross section. The broken lines showthe paths observed when the sextupoles 121 and 122 are turned off, andthe solid lines show the path observed when they are turned on. The pathnear the axis is affected to only a slight extent by the sextupoles.

The sextupoles lie in the exit and entrance planes 5, 6 of the energyanalyzers.

FIG. 10 shows a schematic diagram of a magnetic transfer lens device 20[Sic →20′-Tra] analogous to the electrostatic lenses of FIG. 2. Themagnetic fields of the lenses 22′ and 21′ are generated by coils. Theessential difference between this and an electrostatic transfer lensdevice is an additional rotation of the image by the angle γ, where γ isbased on the position of the image at

$V_{L} = {- {\frac{{ZB}_{2}}{{ZB}_{1}}.}}$

List of Reference Symbols 1 sample 1′ surface of sample 2 first lenssystem 3 entrance plane 4, 4′ central beam bath in the first energyanalyzer 5 exit plane 6 entrance plane 7, 7′ central beam path in thesecond energy analyzer 8 exit plane 9 lens system 10 detector 20, 20′transfer lens device 21, 21′ first transfer lens 22, 22′ second transferlens 23 first intermediate image 24 second intermediate image 25 firstslit diaphragm 26 second slit diaphragm 27 third slit diaphragm 28fourth slit diaphragm 29 image 30, 30′, 30″ first toroidal energyanalyzer 31, 31′, 32″ inner shell 32, 32′, 32″ outer shell 33energy-dispersive plane 34 axis 40, 40′, 40″ second toroidal energyanalyzer 41, 41′, 41″ outer shell 43 energy-dispersive plane 44 axis50a, 50b toroid sector 121 sextupole lens 122 sextupole lens 200 axis ofthe transfer lens device

1. Image-generating energy filter for electrically charged particlessuch as electrons and ions with at least two toroidal energy analyzersarranged in a row, where at least one energy analyzer has a diaphragm atits entrance plane and another diaphragm at its exit plane,characterized in that: a transfer lens device (20, 20′) is locatedbetween the exit plate (5) of the first energy analyzer (30, 30′, 30″)and the entrance plane (6) of the second energy analyzer (40, 40′, 40″),which device has negative lateral magnification V_(L), negative angularmagnification V_(W), image rotation around the angle γ=β−180°, and atelescopic beam path, where its respective deflection angles φ are equaland its energy-dispersive planes (33, 43) are rotated around the angle βwith respect to each other.
 2. Energy filter according to claim 1,characterized in that the transfer lens device (20, 20′) is designed sothat, in the energy-dispersive plane (33), it projects the intermediateimage ZB₁ (23) present at the exit plane (5) of the first energyanalyzer (30, 30′, 30″) with a linear magnification of$V_{L} = {\frac{{ZB}_{2}}{{ZB}_{1}} < 0}$ and with an angularmagnification of${V_{W} = {{- \frac{\alpha_{2}}{\alpha_{1}}} < {0\mspace{11mu}{where}\mspace{14mu} V_{W}V_{L}\sqrt{\frac{E_{2}}{E_{1}} = 1}}}},$rotated around the angle γ, onto the entrance plane (6) of the secondenergy analyzer (40, 40′, 40″) as intermediate image ZB₂ (24), where α₁is the exit angle of the charged particles from the exit plane (5) ofthe first energy analyzer (30, 30′, 30″); α₂ is the entrance angle tothe entrance plane (6) of the second energy analyzer (40, 40′, 40″); E₁is the kinetic energy of the charged particles in the exit plane of thefirst energy analyzer (30, 30′, 30″); and E₂ is the kinetic energy ofthe charged particles in the entrance plane of the second energyanalyzer (40, 40′, 40″), and where the charged particles in the transferlens device (20, 20′) pass through a telescopic beam path.
 3. Energyfilter according to claim 1, characterized in that the energy analyzers(30, 30′, 30″; 40, 40′, 40″) and the transfer lens device (20) arearranged with radial symmetry around the center of the transfer lensdevice.
 4. Energy filter according to claim 1, characterized in that theenergy analyzers (30, 30′, 30″; 40, 40′, 40″) are built with differentdimensions.
 5. Energy filter according to claim 1 characterized in thatthe energy analyzers are spherical sectors (30′, 40′), hemisphericalanalyzers (30, 40), or cylindrical analyzers (30″, 40″).
 6. Energyfilter according to claim 5, characterized in that the energy-dispersiveplanes (33, 43) of the hemispherical analyzers (30, 40) are rotatedaround the axis (200) of the transfer lens device (20) by the angleβ=180°, so that the beam path has a the shape of an “S”.
 7. Energyfilter according to claim 1, characterized in that the transfer lensdevice (20) comprises at least one electrostatic tube lens (21, 22). 8.Energy filter according to claim 1, characterized in that the transferlens device (20′) comprises at least one magnetic lens.
 9. Energy filteraccording to claim 1, characterized in that the transfer lens device(20) comprises at least one multipole lens (121, 122).
 10. Energy filteraccording to claim 1, characterized in that the transfer lens device(20) has at least two lenses (21, 21′) and (22, 22′), and in that theexit plane (5) of the first energy analyzer (30, 30′, 30″) is located atthe focal point of the first lens (21, 21′) and the entrance plane (6)of the second energy analyzer (40, 40′, 40″) is located at the focalpoint of the second lens (22, 22′), where the distance between the twolenses is 2F, where F stands for the focal distance of the lenses (21,22, 21′, 22′).
 11. Use of an energy filter according to claim 1 forelectron microscopes.
 12. Use of an energy filter according to claim 1for time-resolved measuring instruments.
 13. Energy filter according toclaim 2, characterized in that the energy analyzers (30, 30′, 30″; 40,40′, 40″) and the transfer lens device (20) are arranged with radialsymmetry around the center of the transfer lens device.
 14. Energyfilter according to claim 2, characterized in that the energy analyzers(30, 30′, 30″; 40, 40′, 40″) are built with different dimensions. 15.Energy filter according to claim 2, characterized in that the energyanalyzers are spherical sectors (30′, 40′), hemispherical analyzers (30,40), or cylindrical analyzers (30″, 40″).
 16. Energy filter according toclaim 2, characterized in that the energy-dispersive planes (33, 43) ofthe hemispherical analyzers (30, 40) are rotated around the axis (200)of the transfer lens device (20) by the angle β=180°, so that the beampath has a the shape of an “S”.
 17. Energy filter according to claim 2,characterized in that the transfer lens device (20) comprises at leastone electrostatic tube lens (21, 22).
 18. Energy filter according toclaim 2, characterized in that the transfer lens device (20′) comprisesat least one magnetic lens.
 19. Energy filter according claim 2,characterized in that the transfer lens device (20) comprises at leastone multipole lens (121, 122).
 20. Energy filter according to claim 2,characterized in that the transfer lens device (20) has at least twolenses (21, 21′) and (22, 22′), and in that the exit plane (5) of thefirst energy analyzer (30, 30′, 30″) is located at the focal point ofthe first lens (21, 21′) and the entrance plane (6) of the second energyanalyzer (40, 40′, 40″) is located at the focal point of the second lens(22, 22′), where the distance between the two lenses is 2F, where Fstands for the focal distance of the lenses (21, 22, 21′, 22′).